The present invention relates to ferromagnetic particles for higher recording density, particularly to the spindle ferromagnetic alloy particles which are most suitable for short-wave recording for long-time, and which are uniform in particle size and shape, not contaminated by the dendrites and are highly monodispersed particles and as a result, are large in apparent density and have an excellent dispersibility and which have the ratio of the length of the major axis to that of the minor axis (hereinafter referred to as the aspect ratio) of less than 3:1, particularly less than 2:1 and a coercive force of 500 to 1000 Oe, and also relates to the process for producing the same.
Recently, with the progress of the long-time recording, the miniaturizing and the weight-saving of the reproducing apparatus for magnetic recording such as video tape recorder, the demand for the magnetic recording reproducing apparatus and the magnetic recording media such as magnetic tape, magnetic discs, etc., with higher performance and higher recording density has been intensified.
In order to form the magnetic recording media with high performances and higher recording density, it is necessary to improve the magnetic properties of magnetic particles, for example, the coercive force (Hc) and the saturation magnetization (.sigma..sub.s), and their dispersibility in the vehicle and their orientation and loading in a coating medium and to improve the smoothness of the surface of the tape, and to make the residual magnetic flux density (Br) higher and the coating medium thinner. These facts are clearly recognized from the following descriptions, for instance, in "Development of magnetic materials and technique of highly dispersing magnetic particles", page 140, published by SOGO-GIJYUTSU CENTER of Japan (1982).
"In the improvement of the recording density--, it is necessary to increase the residual magnetic flux density Br for securing a predetermined output, and for increasing the residual magnetic flux density Br, the orientation of the magnetic powder into the direction of the magnetic field must be increased and the degree of loading of the magnetic particles must be high.", in page 15 of the same publication, "the important index representing the performance of the magnetic material in magnetic recording is--the recording density, and the improvement of the recording density has been carried out by improving the magnetic head and the recording medium. The improvements which have been carried out in this field are mainly directed to realizing the magnetic layer which is thin in thickness and high in the coercive force(Hc) as the recording medium--.", in page 141 of the same publication, "for a high recording density, it is the most important factor to make the coating medium thin", in page 312 of the same publication, "the condition for obtaining a high recording density while using a recording tape of a coated type is to retain the high output performance with a low noise to the short-wave signals, and for that purpose, it is necessary that both the coercive force(Hc) and the residual magnetic flux density(Br) are large while balancing thereof and thickness of the coating medium is thin" and in page 143 of the same publication, "The amount of floating-up of the head in the case of the head floating-up type such as in the rigid disc device is the major factor of the high recording density, and such a high recording density is possible by decreasing the spacing between the head and the recording medium. However,--in the case of the surface roughness not small enough, the reduction of the output signal occurs and it causes the headcrush due to the chipping of the head. Accordingly,--it is necessary to carry out the finishing of the surface of the coating medium as smooth as possible."
Since these properties of the medium for magnetic recording have a close relationship to the magnetic particles as the magnetic media, the improvements in the properties of the magnetic particles have been strongly demanded.
The relationship between the specific properties of the magnetic media and properties of the magnetic particles are described in detail as follows.
First of all, the residual magnetic flux density(Br) of the magnetic media depends on the dispersibility of the magnetic particles in the vehicle and their orientation and loading thereof in the coating medium.
In order to improve the dispersibility in the vehicle and the orientation and loading in the coating medium, of the magnetic particles, it is demanded that the particles are uniform in size and shape thereof and not contaminated by dendrites and as a result, that of the particles have large apparent density, and that mutual sintering between the particles is prevented.
In the next place, in order to improve the surface properties of the magnetic recording media, it is demanded that the magnetic particles are preferably excellent in the dispersibility and the orientability and preferably small in the particle size. As such magnetic particles, also it is required that the particles are uniform in size and shape thereof and not contaminated by dendrites and as a result, that the particles have large apparent density, and that mutual sintering between the particles is prevented.
In addition, in order to make the magnetic recording media as thin as possible, it is demanded that the magnetic particles are preferably excellent in the dispersibility and the orientability, and are uniform in size and shape and not contaminated by dendrites and that mutual sintering between the particles is prevented, as is clearly seen in the afore-mentioned publication on page 141 "For making the magnetic recording media as thin as possible, it is necessary to make the size of the magnetic particle as small as possible and to give the excellent orientability in the coating medium. Formation of a thin coating medium is related to make a magnetic paint which is excellent in applicability by using the magnetic particles with small oil absorption."
Also, the magnetic particles for use in magnetic recording have been generally obtained by subjecting (1) the goethite particles which are the starting material, (2) the hematite particles obtained by thermally dehydrating the goethite particles or (3) the goethite particles or the hematite particles both of which contain other metal(s) than Fe to thermal reduction in a reducing gas, thereby obtaining the magnetite particles or ferromagnetic iron alloy particles, or further, subjecting the thus obtained magnetite particles to oxidation to form magnetic maghemite particles.
In order to obtain the magnetic particles which are uniform in particle size and shape and are not contaminated by dendrites and are monodispersed particles and are prevented from the mutual sintering between the particles, it is important at first that the particles used as the starting material are uniform in particle size and shape and not contaminated by the dendrites and the entwined particles, and in the next place, how to subject such a starting material to thermal reduction while retaining the original shape and form thereof becomes a large problem.
Hitherto, the most representative known process for producing the goethite particles as the starting material comprises the steps of adding an aqueous solution of an alkali in an amount more than equivalent into an aqueous solution of a ferrous salt, thereby obtaining an aqueous suspension containing ferrous hydroxide and oxidizing Fe(OH).sub.2 in the thus obtained aqueous suspension at a temperature of lower than 80.degree. C. to obtain the acicular goethite particles. The goethite particles thus obtained are contaminated by dendrites and from the standpoint of particle size thereof, it cannot be said that the particles are uniform in size.
Concerning the step of thermal reduction of the goethite particles, in the case where the goethite particles are subjected to thermal reduction to obtain the magnetic particles, by raising temperature of reduction, the magnetic particles having a large saturation magnetization are produced, however, in the case where the temperature of thermal reduction is too high, the deformation of the magnetic particles and the mutual sintering between the particles becomes remarkable.
The causes of the deformation of the particles and the mutual sintering between the particles in the thermal reduction step are explained as follows.
In general, the hematite particles obtained by thermally dehydrating the goethite particles at a temperature around 300.degree. C. retain the original shape of the goethite particles (hereinafter referred to as the skeleton hematite particles), however, there are many pores on the surface of the particles and within the particles formed by dehydration and accordingly, the growth of the primary unit particles has not been sufficient resulting in the poor crystallization.
On the thermal reduction of such hematite particles, because of the rapid growth of unit particles, in other words, because of the abrupt physical change, the uniform growth of unit particles in the skeleton particles hardly occurs and accordingly, it is considered that the deformation of the particles and the mutual sintering between the particles are caused in the part where the rapid growth of unit particles occurs to give the irregularly shaped particles and sintered particles.
In addition, in the step of thermal reduction for obtaining the ferromagnetic alloy particles, rapid volume contraction of the particle due to the change from metal oxide to metal is caused, and such a rapid volume contraction is one of the reasons of the deformation of the particles.
Accordingly, in order to prevent the deformation of the shape of the particles and the mutual sintering between the particles during the thermal reduction of the hematite particles, it is necessary to have the hematite particles of a high crystallinity and a substantially high density, which retain the original shape of the goethite particles, before subjecting the hematite particles to thermal reduction, by making the sufficient and uniform growth of the hematite particles.
As a method for obtaining such hematite particles of a high crystallinity and a high density, a method comprising subjecting the goethite particles to thermal treatment in a non-reducing atmosphere has been known.
Generally, in the case of thermally dehydrating the goethite particles in a non-reducing atmosphere to obtain the hematite particles, by raising the temperature of thermal treatment, the growth of the unit particle becomes more effective resulting in the improved degree of crystallinity of the skeleton particle, however, it has been known that in the case of the temperature of thermal treatment of over 650.degree. C., mutual sintering between the particles is promoted thereby causing the deformation of the particles.
Accordingly, in order to obtain the hematite particles of an improved crystallinity and a substantially high density in which the particle-shape of the goethite particles has been retained, a method has been known wherein the surface of the goethite particles is coated with an organic compound or an inorganic compound which has an activity of preventing the sintering, before the goethite particles are subjected to thermal treatment in a non-reducing atmosphere.
In order to make a high recording density, it is necessary that the coercive force(Hc) of the magnetic recording media is as high as possible, and for that purpose, it is necessary that the coercive force(Hc) of the magnetic particles to be dispersed in the vehicle is as high as possible.
In the present, as the magnetic particles for magnetic recording, mainly the acicular magnetite particles or the acicular maghemite particles are used, and these particles generally have a coercive force of around 250 to 350 Oe.
In addition, it has been known to improve the coercive force of the magnetic particles by adding cobalt to the above-mentioned acicular magnetite particles or acicular maghemite particles, thereby obtaining the magnetic particles of a coercive force of around 400 to 800 Oe. However, since the saturation magnetization(.sigma..sub.s) of the thus obtained magnetic particles is 70 to 85 emu/g, in the case of painting such magnetic particles as a magnetic recording media the saturation magnetic flux density (Bm) is at most 2000 Gauss.
For the purpose of obtaining the magnetic recording media of a high density, it is necessary to use the magnetic particles of a high coercive force(Hc) and a large saturation magnetization(.sigma..sub.s), and those ferromagnetic iron particles and those ferromagnetic alloy particles both of which have a high coercive force(Hc) and a large saturation magnetization(.sigma..sub.s) are attracting the attention and are in practical use.
Saturation magnetization of the ferromagnetic iron particles or the ferromagnetic alloy particles is around 110 to 170 emu/g, and the coercive force thereof is around 1000 to 1500 Oe, and efforts for still improving the coercive force are still carried out.
The ferromagnetic iron particles and the ferromagnetic alloy particles are obtained by subjecting the acicular particles of ferric iron oxide hydroxide, the acicular particles of ferric oxide or the particles thereof containing a different kind of metal other than Fe as the starting material to thermal reduction in a reducing gas.
However, on the other hand, it has been known that there is a close relationship between the coercive force(Hc) of the magnetic recording media and the performances of the magnetic head, and that in the case where the coercive force(Hc) is too high, since.the electric currency for recording becomes too high, in the ferrite head which is most broadly used in the present, the pole tip is magnetically saturated due to the low saturation magnetic flux density (Bm) of the head core, and it becomes impossible to sufficiently magnetize the magnetic recording media.
The above-mentioned facts are clearly seen in the description of Technical Research Report of DENSHI TSUSHIN GAKKAI (The Inst. of Electronics and Communication Engineers of Japan), MR82-19 (1982), page 19.
"In order to record the signals on a high coercive force(Hc) tape (Hc being 1000 to 1500 Oe), a video head using a core of a high saturation magnetic flux density (Bm) is required, and in the case of using a video head of the conventional MnZn ferrite, it is anticipated that the high coercive force(Hc) tape cannot be sufficiently magnetized due to the occurrence of magnetic saturation caused by the deficiency of the saturation magnetic flux density (Bm)."
As has been described above, the coercive force(Hc) of a magnetic recording media and the performances of the magnetic head are in a close relationship, and for that reason, in the apparatus for reproducing the magnetic records which uses the magnetic recording media prepared by using the magnetic particles of the coercive force of less than 1000 Oe such as the acicular magnetite particles, the acicular maghemite particles, the acicular particles of ferric oxide having the surface layer thereof modified by cobalt, etc., a MnZn ferrite head is generally used.
On the other hand, in the apparatus for reproducing the magnetic records which uses the magnetic recording media prepared by using the magnetic particles of the coercive force of larger than 1000 Oe such as ferromagnetic alloy particles, a head prepared by a material of a high saturation magnetic flux density(Bm) such as sendust head, amorphous head, thin film head, etc. is generally used.
However, in the head prepared by one of the above-mentioned materials, a new problem such as the head wearing due to the contact of the magnetic recording media with the head for recording and the head for reproducing has been caused, the problem having been relatively out of the question in the cases where the ferrite head is used.
Accordingly, the ferromagnetic iron particles or the ferromagnetic alloy particles provided with a large saturation magnetization(.sigma..sub.s) and a suitable coercive force(Hc) for use in the reproducing apparatus of magnetic records with a ferrite head have been demanded.
Namely, as has been discussed above, the coercive force(Hc) of a magnetic recording media should be well balanced in consideration from the two sides, i.e., a high recording density and material for the magnetic head.
At present, as the magnetic recording media used in the reproducing apparatus of magnetic records in which the most popularized ferrite-head has been incorporated, those of a suitable coercive force(Hc) by which a high recording density is possible and the pole tip saturation problem of the ferrite-head can be avoided, namely, the coercive force of 500 to 1000 Oe have been demanded. In order to obtain a magnetic recording media of a coercive force(Hc) of around 500 to 1000 Oe, it is necessary that the magnetic particles to be dispersed in the vehicle has a coercive force of 500 to 1000 Oe.
Further, the ferromagnetic alloy particles, which are not contaminated by dendrites and are monodispersed each other, and as a result, are large in apparent density and excellent in dispersibility in the vehicle, and have a coercive force of from 500 to 1000 Oe, are most eagerly demanded as the magnetic particles by which a high magnetic recording density is possible and pole tip saturation problem of the ferrite-head can be avoided.
However, the goethite particles, which are the starting material of the above-mentioned ferromagnetic alloy particles, obtained by the afore-mentioned, known process are contaminated by the dendrites, as has been stated, and cannot be said that they are uniform in particle size, and they are acicular-shape with the axial ratio of larger than 10:1.
The ferromagnetic alloy particles obtained by subjecting such goethite particles which are contaminated by the dendrites and not uniform in particle size to thermal reduction in a reducing gas are also contaminated by the dendrites and not uniform in particle size. In the case where a magnetic recording media is prepared while using such ferromagnetic alloy particles, the dispersibility thereof in the vehicle and the orientation and loading thereof in the coating film are poor resulting in the reduced residual magnetic flux density.
Accordingly, the present inventors paid an attention to a process for producing the goethite particles by blowing an oxygen-containing gas into an aqueous suspension containing FeCO.sub.3, which has been obtained by reacting an aqueous solution of a ferrous salt with an alkali carbonate(refer to Japanese Patent Application Laying-Open No. 50-80999).
By actually carrying out the above-mentioned process, spindle goethite particles which are uniform in particle size and shape and not contaminated by the dendrites are obtained.
The spindle ferromagnetic alloy particles obtained by subjecting the thus obtained, spindle goethite particles which are uniform in particle size and shape and not contaminated by dendrites as the starting material to thermal reduction are also uniform in particle size and not contaminated by the dendrites, however, the coercive force of the thus obtained ferromagnetic alloy particles is larger than 1000 Oe. The fact is clearly seen, for instance, in the description of Example 3 of Japanese Patent Application Laying-Open No. 53-10100. Namely, Example 3 thereof discloses the process for obtaining the ferromagnetic alloy particles comprising the steps of blowing an oxygen-containing gas into an aqueous suspension containing FeCO.sub.3 obtained by reacting an aqueous solution of a ferrous salt with an alkali carbonate, thereby obtaining spindle goethite particles and subjecting the thus obtained, spindle goethite particles to thermal reduction. The coercive force of the thus obtained ferromagnetic alloy pparticles is 1020 to 1165 Oe.
Accordingly, the offer of a process for producing spindle ferromagnetic alloy particles which are not contaminated by dendrites, are uniform in particle size and shape and have the coercive force of 500 to 1000 Oe is also strongly demanded.
In consideration of the above-mentioned facts, the present inventors have studied the conditions for obtaining the magnetic particles which are uniform in particle size and shape and not contaminated by dendrites, and have a large value of saturation magnetization(.sigma..sub.s) and a coercive force of about 500 to 1000 Oe, and as a result of the present inventors' studies, they have attained the present invention.